As a technique for prevention of dusting of powder such as cement that tends to scatter dust upon handling or during transportation or storage, a method of adding a fibril-forming polytetrafluoroethylene (hereinafter polytetrafluoroethylene is simply referred to as PTFE) to the powder having dusting property, stirring them to exert compression/shear force so as to form PTFE fibril from fibril-forming PTFE thereby suppressing dusting has been developed.
The dusting-preventive treating technique described above is disclosed, for example, in Japanese Published Examined Patent Application No. 24872/1993 (JP-H05-24872 B4). When a fibril-forming PTFE is added to a cement powder and fibril-forming treatment of exerting compression/shear force is applied by using the technique disclosed in this patent publication, since the cement powder is in a state of being covered and agglomerated in cobweb-like nets of PTFE fibrils, a dusting-preventive cement with no scattering of cement powder in atmospheric air can be obtained.
Further, Japanese Published Examined Patent Application No. 32877/1977 (JP-S52-32877-B4) discloses a technique of mixing a fibril-forming PTFE with a powdery material and exerting compression/shear force to the mixture at a temperature in the range of 20 to 200° C., thereby forming PTFE fibril and suppressing dusting of the powdery material.
As described above, occurrence of dusts can be prevented by applying the dusting-preventive treatment to the cement powder or the like based on the technique as disclosed in Japanese Published Examined Patent Application No. 24872/1993 and Japanese Published Examined Patent Application No. 32877/1977.
The methods of preventing dusting by the nets of the PTFE fibrils described above involve a drawback in that strong air pressure, suction force or shearing force exerts on the dusting-preventive powdery product when it is transported under pressure or transported under suction by a pneumatic transportation apparatus used generally for powder transportation or when transported at a high speed by a screw conveyor or a rotary feeder at high speed rotation as a powder transportation method, the PTFE fibrils is partially broken or a portion of powdery particles is detached from the nets of the PTFE fibrils to remarkably lower the dusting-preventive performance. Accordingly, in the transportation step of the dusting-preventive powder products in a production site has a problem that it is inevitable to use a transportation machine not exerting strong external force to the product such as a belt conveyor and transportation of the product undergoes restriction to some extent in view of the apparatus and in view of the quantity. In addition, even when the dust-forming powder deteriorated with the dusting-preventive performance is stirred at a temperature of 20 to 200° C. again to exert compression/shear force, the dusting-preventive performance can not be recovered. Accordingly, for the recovery of the dusting-preventive performance, it is necessary for again adding an appropriate amount of fibril-forming PTFE and stirring them at a temperature in the range of 20 to 200° C. to exert compression/shear force.
A subject intended to be solved in accordance with this invention is to provide a dusting-preventive treating method capable of advantageously recovering or improving the dusting-preventive performance of the dust-forming powder whose dusting-preventive performance has been lowered during transportation, by a relatively simple step and with economical advantages.